A torque converter has heretofore been known as having a pump wheel, a turbine wheel and a stator wheel rotatably disposed between the turbine wheel and the pump wheel. With the torque converter of such a related art, the stator wheel is connected to a non-rotating member via a one-way clutch with no provision of variable displacement characteristic. In general, the torque converter has a fluid characteristic that needs to have a high capacity (capacity coefficient) with high directivity for fuel consumption. With such a related art structure, the fluid characteristic is uniquely determined with shapes of the pump wheel, the turbine wheel and the stator wheel. Therefore, the torque converter exhibits identical fluid characteristics regardless of running patterns, resulting in limitations in improving fuel economy performance and power performance at once.
As disclosed in Patent Publication 1, on the contrary, a proposal has been made to provide a variable displacement type torque converter with a structure incorporating brake means between the turbine wheel and the non-rotating member, under which brake torque of the braking means is adjusted to allow the capacity to be variable. With such a structure, adjusting brake torque of the braking means results in a capability of varying a torque ratio and a capacity coefficient of the torque converter in a continuously variable mode or in a multi-stage mode. This enables an optimum torque ratio and capacity coefficient to be determined depending on a driving condition and a running condition, resulting in an increase in running performance of a vehicle.    [Patent Publication 1] Japanese Patent Application Publication No. 1-169170
With the variable displacement type torque converter of such a related art, however, the stator wheel was merely controlled in rotation within a range of a negative rotating direction opposite to a rotating direction of the pump wheel, resulting in limitations in an upper limit value of torque ratios and a lower limit value of the capacity coefficient that could be obtained with such a structure. Accordingly, the torque ratio of the torque converter could not be necessarily and adequately increased to vary the capacity coefficient to a lower level depending on the driving condition and the running condition, resulting in a difficulty of adequately increasing power performance of the vehicle.